Surface Activity of Magnesium During GaN Molecular Beam Epitaxial Growth
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ex situ by hydrogen etching [6]. The surface oxide is removed by Si pre-deposition and desorption [7] till a √3×√3R-30 ° reconstructed surface is obtained. Immediately after this, the substrate is brought to growth temperature and growth is initiated. Growth temperatures are in the range 600–700 ° C. Growth conditions are usually Ga-rich, for which we observe a streaky 1×1 RHEED pattern during growth. However, for our studies of the surfactant effect of Mg, we have grown films in the N-rich regime before Mg exposure. The hallmark of the N-rich regime is a spotty, bright RHEED pattern. Ga and Mg fluxes are produced by effusion cells, while an RF-plasma is used to excite N2 molecules thereby facilitating reaction. Mg exposure is done during growth and is seen not to have any effect when performed during growth interrupts. This may have to do with the short residence times for Mg atoms on the GaN surface at the growth temperature. During growth, atoms on the surface are mobile enough for Mg to get incorporated in the crystal surface immediately upon their arrival, whereas even with prolonged exposures during interrupts little Mg sticks to the surface. Following growth, surfaces are characterized using in situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM), as described elsewhere [8]. RESULTS Surfactant Behavior of Mg GaN films grown in the Ga-rich regime show a smooth two-dimensional (2-D) morphology. In Fig. 1(a), a RHEED pattern and AFM image obtained for a film grown under these conditions are displayed. The RHEED pattern is dim and streaky, since the growth is 2-D and at all times the surface is covered with excess Ga (at least 2 disordered ML) [8]. The AFM image displays deep pits and trenches (> 150 Å deep) which are characteristic of the columnar growth in the MBE process. In between these pits, however, the film shows a flat, 2-D morphology with atomic steps faintly visible. As the Ga flux becomes smaller, the RHEED pattern becomes brighter and eventually in the N-rich region of growth, the RHEED pattern begins to get spotty as shown in the inset in Fig. 1(b). The morphology of the film in the N-rich regime is shown in Fig. 1(b), and the roughening is seen as the granular appearance of the film between the deep pits.
Figure 1 (a) AFM image of a film grown under Ga-rich conditions, (b) grown under N-rich conditions. In the insets we see corresponding RHEED patterns in the (11 2 ) azimuth.
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During growth, films are briefly exposed to a Mg flux. A surfactant effect of Mg is seen on these Ga-polar films in the N-rich regime. Exposing the growth surface to as little as 0.2 ML of Mg under Ga-poor conditions leads to a reversal of the RHEED pattern from spotty to streaky. Also, when the growth is made very N-rich by reducing the Ga flux to about one half of that at the transition point, exposure to Mg often produces a streaky 2×2 pattern. At typical sample temperatures during growth (625 ° C for the sample shown below), the sticking coefficient of Mg on the GaN sur
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